1. Field of the Invention
The present invention relates to an optical recording medium, and more particularly to a method of detecting and compensating for defocus and tilt of the optical recording medium.
2. Description of the Related Art
Generally, an optical recording medium is classified into three types, that is, a read only memory (ROM) type, a WORM type that is writable once only, and a rewritable type that is repeatedly writable.
Examples of the repeatedly rewritable optical recording medium are on optical disc, a rewritable compact disc (CD-RW) and a rewritable digital versatile disc (DVD-RW, DVD-RAM, and DVD+RW).
A rewritable optical disc such as the DVD-RAM has signal tracks composed of lands and grooves, and tracking control can be performed even in a blank disc on which no information signal is recorded. In order to heighten the recording density, the information signal is recorded on the lands and the grooves, respectively.
In manufacturing the optical disc, warp may occur during a resin-injection and hardening process, and eccentricity may occur despite the center hole in the disc. Eccentricity may also occur due to the deviation of the center hole though the tracks are accurately written on the disc with a pitch of a predetermined standard and in a spiral shape. Accordingly, the disc rotates eccentrically, and thus the center axis of a driving motor may not completely coincide with the center of the track.
Since this makes it difficult to accurately read the signal recorded on a desired track, a standard for the CD and DVD is determined with respect to the amount of deviation, and a tracking servo is performed so that an optical beam constantly follows the desired track even if eccentricity occurs.
The tracking servo produces an electric signal corresponding to a beam trace state, and corrects the position of the beam by moving an object lens or an optical pickup body in a radial direction according to the electric signal, so that the track is accurately followed.
Meanwhile, deviation of the beam from the corresponding track occurs due to the inclination of the disc, as well as the eccentricity of the disc. This disc inclination may occur due to a mechanical problem such as a placement error when the disc is placed on a spindle motor. At this time, focusing does not perpendicularly accord with tracking, but is distorted. The inclined state of the disc is called a tilt.
Such a tilt matters little in case of a CD having a wide track pitch and thus having a big tilt margin. Here, the tilt margin refers to the amount of inclination that can be compensated for. However, the DVD, which is a high-density optical application device, has a narrow track pitch, and thus has a small radial tilt margin for a zitter. Accordingly, if only a small tilt occurs, i.e., if the disc is inclined only slightly, a detrack causing the beam to go over to a neighboring track occurs, and this cannot be sufficiently corrected by the tracking servo alone. In other words, if the beam is located on the center of the track even though the beam goes over to the neighboring track, the tracking servo may judge that the track is accurately followed.
In this case, data cannot be accurately read out from the disc during reproduction. Also, data cannot be accurately written on the corresponding track during recording, and this causes dual distortion to occur in the reproduced data.
According to a method for solving the above-described tilt, a dedicated tilt sensor for detecting the tilt, for instance, a light-receiving element for tilt detection is provided in the pickup, and detects the tilt of the disc. This method, however, has a drawback in that an increase in the size of the set results in a loss of efficiency.
Meanwhile, in order to record information or reproduce the recorded information, the optical recording/reproducing apparatus performs focus control through the optical pickup.
During focus control, i.e., focus servo, if an optical focus deviates from the disc surface (hereinafter referred to as defocus), the quality of the recorded or reproduced data deteriorates, and this causes the operation of the system to become unstable.
Accordingly, the focus servo is performed in a manner such that the optical pickup moves up and down by driving a focus actuator in the optical pickup to follow upward and downward movement of the rotating optical disc. Specifically, the focus actuator keeps the distance between the object lens and the optical disc constant by driving the object lens up and down, for example, in the direction of the focus axis, according to a focus error signal.
Meanwhile, in the optical disc such as the DVD-RAM, whereby data can be recorded on both the lands and the grooves, the focus offsets are different due to the depth difference between the land and the groove, and thus defocus may occur even if the focus error signal is zero.
Specifically, since the offset has not been adjusted due to the depth difference between the land and the groove even if focusing is performed, focusing may not be effected when it is applied to the track of the groove as it is. Also, even in the event that focusing is performed on the track of the groove, defocusing may occur on the track of the land due to the depth difference between the land and the groove.
At this time, the defocus state cannot be identified only by the focus error signal, and this causes the zitter characteristic to deteriorate and a bit error rate (BER) to become larger. If the recording is performed in this state, the recording characteristics of the land and the groove may be changed, so that the quality of the recorded or reproduced data deteriorates, and thus the operation of the system becomes unstable in the same manner as described above.